A glow plug, for assisting the start of an internal combustion engine, is known which integrally includes a combustion pressure sensor for sensing a combustion pressure of an internal combustion engine. In such a glow plug, a heater member, that is disposed at a top end side thereof and that generates heat when energized, is exposed inside the internal combustion engine. The combustion pressure acting on the heater member is transmitted to a piezoelectric element (i.e., a pressure-sensing element) for sensing a combustion pressure via a center shaft. The piezoelectric element is accommodated in a cylindrical casing (housing) at a rear end side of the glow plug in a state where the piezoelectric element is pre-loaded (See, for example, Japanese Laid-Open Patent Publication No. 2004-278932).
The force based on the combustion pressure is transmitted to the piezoelectric element via the center shaft that extends in an axial direction of the glow plug, and thus acts in the axial direction of the glow plug. On the other hand, when the swinging generated by the vibration of the internal combustion engine itself, onto which the glow plug is assembled, causes the force containing the component in the perpendicular direction to the axis to be applied to the center shaft, a situation may arise where the center shaft is inclined relative to the axial direction of the glow plug and is vibrated. If this situation arises, a force in a direction different from the direction along which the piezoelectric element originally undergoes the influence of the combustion pressure is superimposed onto the force generated by the combustion pressure, and this superimposed force is applied to the piezoelectric element. There may be a case where this superimposed force produces a noise and exerts an influence on a sensor output.
In order to reduce the influence of noise such as described above, as is the case of Japanese Laid-Open Patent Publication No. 2004-278932, a sleeve with a flange (an outwardly protruding member) is integrally provided to the center shaft, and the center shaft is passed through the inner hole of a ring-shaped piezoelectric element, and the piezoelectric element is pre-loaded between the flange portion and the top-facing end surface of the inner wall of the case. In this manner, for example, even if a force in a rocking direction applies a force in a compressing direction to one side of the piezoelectric element that is partitioned therefrom by the plane perpendicular to the rocking direction, including an axis so as to generate a positive charge, whereas a force in a stretching direction opposite to the compressing direction is applied to the opposite side so as to generate a negative charge, the charges generated by the rocking direction are equal between one side and the other side partitioned from each other by said plane, so that they can compensate for each other. Thus, these charges can compensate for each other within the piezoelectric element, and as a result, only a force based on the combustion pressure can be detected.
However, the combustion sensor of the glow plug as described above has a structure in which a slit-like opening for taking out an output from the piezoelectric element is formed on a portion of its case, and the output is connected to the signal line outside the case so as to be electrically connected to the external circuit of the glow plug. As described above, the piezoelectric element is pre-loaded between the flange portion of the sleeve and the top end surface of the inner wall of the case. At the portion of the top end surface formed with the opening and at the portion of the same top end surface continued to the opening which is formed on the side wall of the case and extends from the axis toward the radial outward periphery, the case has low rigidity due to the presence of the opening. If the rigidity of the case is partially low, and thus the rigidity of the entire case is non-uniform, the pre-load exerted by the top end-facing end surface of the inner wall of the case to the piezoelectric element may also accordingly be non-uniform. In this case, when a force is applied in a rocking direction, there is a danger that a difference may arise in the absolute amounts of charges generated at one and the other sides of the piezoelectric element partitioned from each other by said plane, and that such a difference may cause noises.
The present invention has been made to solve the problems described above, and an objective thereof is to provide a combustion pressure sensor capable of reducing noises caused by force in a rocking direction, by making uniform the stress exerted on the piezoelectric element; and a glow plug including the same.